The overall goal of this proposed research is to investigate the effects of prolonged hypothermia and the reinstitution of euthermia on the structure of myocardial tissue. A detailed morphometric analysis at tissue, cellular and subcellular levels will be made of left and right ventricular myocardium of rats and thirteen-striped ground squirrels (Citellus tridecemlineatus) exposed to and recovering from profound hypothermia. The ground squirrel will also be studied during the preparative, hibernating, arousal and active phases of its annual cycle. Measurements will include the determination of cell volumes, numbers and ultrastructural characteristics of myocytes, interstitial cells and the specialized muscle cells of the conducting system as well as the size and distribution of myocardial capillaries and their endothelial ultrastructure. These characteristics will be measured in three regions across the ventricular wall in order to evaluate the heterogeneity between endocardial and epicardial zones. Quantitative results will be correlated with autoradiographic and biochemical assays of protein synthesis in order to compare structural changes with the rates of incorporation of newly synthesized molecules into tissues components during growth and turnover of organgelles. In addition, morphometric measurements will also be made of coincident alterations in the wall structure of the thoracic aorta. These studies should lead to a better understanding of tissue responses to hypothermia and the species and seasonal differences that allow a greater hypothermic survival capacity in the hibernator.